A focused ultrasound method for simultaneous diagnostic and therapeutic applications - a simulation study

Similar to other therapeutic methods, ultrasound surgery requires an imagin g modality to monitor the extent of tissue damage during treatment. In this paper, we have considered the method of ultrasound-stimulated acoustic emi ssion (USAE) that uses two ultrasonic beams at high frequency (1.7 MHz) (sa me as that used for ablation) to locally excite the tissue by generating a low-frequency (1-50 kHz) radiation force. Recording of the tissue response at several locations yields an image. The amplitude of the tissue response depends on the mechanical and acoustic tissue properties, namely its stiffn ess and absorption. These two properties were initially hypothesized to hav e counteractive effects on the response amplitude, i.e., the amplitude shou ld increase with absorption and decrease with stiffness. To check this hypo thesis as well as the degree to which these properties influence the respon se, finite-element simulations of a uniform lesion formed inside a homogene ous medium were used. The results show that, as expected, the displacement amplitude decreased with increasing lesion stiffness at lower frequencies ( except at resonance) while, contrary to our initial hypothesis, it increase d with stiffness at relatively higher frequencies (>22 kHz). At resonance, a frequency upshift occurred with increasing stiffness but was found to be highly spatially variant and system dependent, i.e., not yielding a uniform lesion response when imaged. On the other hand, the absorption increase le d to a uniform linear increase of the mechanical response amplitude of the lesion. Therefore, at higher frequencies, increase of the two parameters ha d a synergistic effect on the tissue response to the applied radiation forc e. This study showed that relatively higher frequencies constitute the opti mal range in the use of USAE for coagulation monitoring. A preliminary expe rimental verification in vitro is also provided.